Variable Valve Timing (VVT) Parte 2

Variable Valve Timing (VVT) Parte 2

Rover's unique VVC system
British car maker Rover introduced its own VVT system called VVC (Variable Valve Control) on MGF in 1995. Many experts regarded it as the best VVT system at the time. The VVC can continuously vary the duration of intake valve opening from a minimum 220 degree (crank angle) to a maximum 295 degree. This is unlike cam-phasing VVT systems, which can only shift the point of valve opening forward or backward but have nothing to do with the duration of opening. As a result, the VVC enables higher flow into the combustion chambers at high rev, benefiting high-end power output. On the other hand, unlike cam-changing systems, its adjustment of valve opening duration is continuous, thus mid-range torque is optimized. This make it a better compromise between power and flexiblity than either systems. The following diagram shows its valve timing:

To realize this continuous variation of valve opening duration is a big technical challenge. At high rev, the duration of intake valve opening shall be lengthened, while the duration of intake valve closing shall be shortened. Therefore, the intake camshaft has to rev slower just when the cam lobe is acting on the intake valve. Once the valve is closed, the camshaft has to speed up to shorten the valve closing duration. In the next cycle, the camshaft has to slow down again when the intake valves open, so forth. How to realize such a non-constant, pulsation-like camshaft rotation speed ?

The Rover VVC system uses a very complicated mechanism to implement that. It is difficult to understand, but in essence it utilizes the special property of eccentric drive wheel. Because an eccentric drive wheel rotates about an off-center shaft, if you turn its outer drive ring at constant speed, the shaft will rotate at non-constant, pulsation-like manner. The speed difference depends on the distance between the shaft and the wheel center, i.e. the longer the distance, the larger difference of rotational speed. The VVC uses a slidable shaft to vary this distance hence the speed difference.

The problem is, a camshaft serves multiple cylinders, which have contradicting requirements. For example, cylinder 1 is working at intake stage while other cylinders have their intake valves closed. Suppose the engine is running at high rev, cylinder 1 calls for a slower rotation of intake camshaft while other cylinders need quicker rotation. As a result, the VVC cannot adopt a single intake camshaft like conventional engines. In fact, it needs 4 camshafts for a 4-cylinder engine ! The right hand side picture above shows the 4 camshafts are arranged in 2 groups. Each group has a rigid camshaft (for the inner cylinder) running within a hollow camshaft (for the outer cylinder). Each group is driven by a double-VVC actuator which has 2 drive rings to actuate the 2 camshafts. Because the two groups of camshafts are not connected at all, an additional drive belt has to be introduced. To save space and weight, Rover simply uses the exhaust camshaft to drive the other intake camshaft group.

As you can see, the VVC is a very sophisticated kind of engineering. Compare the Rover 1.8 VVC engine with its non-VVC version, its output is lifted from 120 hp to 145 hp, while maximum torque is improved from 122 to 128 lbft. On the down side, its complexity means higher costs. A four-cylinder engine needs 2 VVC actuators. A V6 engine even needs 4 of them. And then there are the more complicated camshafts and drive belts. These disadvantages prevent it from becoming popular. Following the demise of Rover, the VVC also came to the end.
       

Advantage	Continuous variation of intake valve opening duration improves power and flexiblity.
Disadvantage	Complex mechanism thus expensive; lack of variable lift means not ultimately as powerful as cam-changing VVTs.
Who use it ?	Rover 1.8 VVC engine on MGF, Caterham and Lotus Elise 111S.

Continuous Variable Valve Lift (CVVL)

The earliest variable valve lift systems like Honda VTEC vary valve lift by switching between slow and fast cams at a threshold point. Such discrete mechanism not only creates a step in the power curve (which is perceived as “unrefined”) but its breathing is also a compromise. An ideal variable valve lift (VVL) system should be capable of varying valve lift continuously according to rev, i.e., the higher the rpm, the higher lift is required. Compare with a fixed valve lift compromised for mid-range rev, VVL enhances power at high rev by supplying the engine more air to breath. At low rpm, its reduced valve lift speeds up the air flow, improving air / fuel mixture thus translate to better fuel economy and cleaner emission. Moreover, car makers can make use of CVVL to regulate engine output, thus eliminate the need of throttle butterfly and reduce so called “pumping loss” (see more info in our Green Technology section).

  Example: BMW Valvetronic

Debuted in BMW 316ti Compact in 2001, Valvetronic was the first continuous variable valve lift mechanism made into production. Instead of enhancing power, the goal of Valvetronic was to reduce fuel consumption. According to the position of throttle pedal, it regulates engine output by varying the depth of valve lift. This mean conventional throttle butterfly can be disabled thus reduces pumping loss. Overall, BMW achieved 10% reduction in fuel consumption with Valvetronic.

Compare with a conventional engine, Valvetronic adds an electric motor, an eccentric shaft and at each intake valve an intermediate rocker arm. The intake camshaft acts on the intermediate rocker arms through roller bearings. When the driver calls for more power, the electric motor turns the eccentric shaft, which pushes the intermediate rocker arms and in turn pushes the valve to open deeper. You can understand its theory easily by reading the illustrations below.

 Although Valvetronic is effective to reduce fuel consumption at part-load, it does not benefit top end power at all, because its additional components result in additional friction and inertia, thus limit the engine’s revvability. This is why BMW has never applied Valvetronic to its high-performance M-power engines. Another disadvantage is its size, which occupies a lot of space above the cylinder head.

Advantage	Reduce fuel consumption
Disadvantage	Large size, additional friction and inertia thus not suitable to high-revving engines
Who use it ?	BMW inline-4, inline-6, V8 and V12

Example: Nissan VVEL

Nissan introduced its Variable Valve Event and Lift (VVEL) in 2007 as the world's second CVVL system. The first application was on the VQ37VHR V6 engine of Skyline Coupe (Infiniti G37). Compare with BMW's Valvetronic, Nissan's system is more compact, involve less parts and less energy loss, therefore it is suitable to high-performance engines.

Though saying VVEL employs less parts, it is still a complicated design and not easy to understand. The above diagrams show its internal construction, which doesn't look like conventional valve gears at all. The VVEL does not use conventional intake camshaft. Each valve is actuated by a cam which is pivoted on - but not fixed to - the camshaft. While conventional cams rotate about the camshaft, the cam in VVEL swings up and down reciprocatingly, this is why it does not need a symmetric profile. Its movement is driven by the camshaft via a series of components, i.e. eccentric cam (which is fixed at the camshaft), link A, rocker arm and link B. Isn't it very complicated ? The following animation will help you understand how it operate:

 High lift

Low lift

How does VVEL vary valve lift ? This is implemented by the eccentric control shaft inside the rocker arm. By rotating the eccentric control shaft, the position of rocker arm is shifted, changing the geometry of Link A and B, then the swing angle of cam. The swing angle of cam determines the degree of valve lift, as you can see from the above diagrams.

Nissan said VVEL saves 10% fuel at light load due to the reduced role of throttle butterfly (it does not eliminate throttle completely), but it did not specify how much gain in horsepower. The VQ37VHR produces 8 percent more horsepower than its predecessor, the non-VVEL VQ35HR. Taking its increased displacement and compression ratio into account, VVEL seems to contribute little to top end power. This is because its benefit in breathing efficiency is largely cancelled out by the additional friction of VVEL components. However, the VQ37VHR engine can rev up to 7500 rpm, proving that VVEL does not compromise top end performance like BMW Valvetronic.

Advantage	Enhanced power at high rev. Save fuel by eliminating throttle butterfly.
Disadvantage	Mechanism still complicated, bulky and expensive.
Who use it ?	Nissan VQ37VHR V6

  Example: Toyota Valvematic

Toyota joined the CVVL club in 2008 with its Valvematic technology. Compare with BMW Valvetronic and Nissan VVEL, Valvematic seems better in many aspects: its construction is relatively simple; It is compact and does not increase the height of cylinder head; Most importantly, it adds little inertia and friction, thus does not compromise top end power. Toyota claims it improves 10% in power output while reduces 5-10% fuel consumption in regular driving.

Valvematic employs an intermediate shaft (blue part in top left picture) to achieve continuous variable valve lift. The intermediate shaft has an actuating member for each cylinder. Each actuating member is made of two finger followers laminating a roller bearing member (top right picture). The finger followers can rotate in relation to the roller member by means of internal gear threads and an electric motor attached to the end of the intermediate shaft. Note that the gear threads of roller member and finger followers are in opposite direction. This mean when the shaft swivels, the roller member and finger followers will move in opposite direction, moving either apart or closer together. In this way, the axle angle between them can be varied infinitely by the electric motor.

Now see the picture below. The intake valve is actuated by camshaft via intermediate shaft. More precisely speaking, the camshaft acts on the roller member of intermediate shaft, transferring the movement to both finger followers, then towards the roller rocker arms and eventually to the intake valves.

As you can see from the picture above, when the finger follower is set at narrow angle in relation to the roller member, it results in low valve lift. When the angle of finger follower is increased (picture below), the valve lift is also increased. In this way, Valvematic can vary valve lift by adjusting the angle of finger followers. In the first 2.0-liter Valvematic engine, lift can vary from 0.97mm to 11mm. The former saves the need of throttle butterfly thus reduce fuel consumption in part load. The high lift enables stronger top end power. Take the 2.0-liter Valvematic engine as example again, it produces a maximum 158 horsepower, up from 143 hp of the regular dual-VVT-i version.

Advantage Increased power, reduced fuel consumption, compact size. Disadvantage - Who use it ? Toyota 1.6 / 1.8 / 2.0 Valvematic engines

Electrohydraulic valve control

  Example: FIAT Multiair
Some variable valve control systems, such as BMW Valvetronic, Nissan VVEL and Toyota Valvematic, are capable to vary valve lift infinitely according to needs. In addition to continuous variable cam phasing, they seemed to be very capable already. However, these systems are still far from perfect. An ideal valve control system should allow unlimited flexibility of valve strategy - in other words, any valve lift at any time - to achieve the desired combustion effect. No mechanical systems can ever achieve that function. Therefore some consultants and suppliers are working on fully electromagnetic valvegears. However, until now such technology still faces technical challenges difficult to overcome, such as size / weight, costs, reliability and energy consumption. Instead of that, the powertrain engineers at FIAT group developed a more practical, but still very flexible enough, type of variable valve control mechanism. They call it "Multiair". The first application is to be introduced to the 1.4 FIRE engine in 2009, then follow by a new 900cc twin-cylinder engine.

Unquestionably, Multiair is the most flexible kind of VVT system until now. From the graphs below you can see it enables at least 5 different types of intake valve strategies to suit different running conditions. Apart from the usual transition between long duration and short duration, high lift and low lift, it also allows late valve opening, early valve closing and, wow, multiple valve lifts during an intake stroke !

Now let us see its mechanism. The Multiair system initially works with SOHC 4-valve construction because its additional electrohydraulic components occupy the space originally left for intake camshaft. While the single camshaft operates the exhaust valves directly in conventional way, it operates intake valves via a series of components: roller rockers ----> hydraulic pistons ---> hydraulic chambers (which incorporate electronic-controlled solenoid valves) ---> hydraulic valve actuators. This mean the actuation is implemented by a combination of mechanical and hydraulic means.

Normally, when the solenoid valve is de-energised and closed, the oil cannot enter the hydraulic chamber, thus it flows directly from hydraulic piston to valve actuator. You can see this hydraulic link as a solid body because the oil has no where to escape. Therefore the intake valve movement follows exactly the intake cam lobe profile. As the intake cam profile is designed to favour high power (i.e. high lift and long opening duration), this strategy is suitable for high rev running. (Fig 1)



When the solenoid valve is energized, it opens and allows the oil to flow into the hydraulic chamber. As a result, no oil will flow to the valve actuator, thus the intake valve will close under the force of its rebound spring. In this way, Multiair can shut down the intake valves at any desired instant. (Fig 3 and Fig 4)

Suppose the intake valve has closed for a while, then the solenoid valve of hydraulic chamber is closed again. What will happen ? In this case, oil will flow directly to valve actuator again, thus the intake valve will follow the cam profile and open again. However, as some time and oil volume has already "lost" (at the hydraulic chamber) during the solenoid valve opening, the valve lift will be reduced. The degree of reduction depends on the instant of solenoid valve closure. The later the solenoid valve close, the lower valve lift will be obtained. In this way, Multiair can vary the lift and opening duration of intake valves. (Fig 2)

Now let us see the above valve lift graphs again:

Fig 1 is suitable for high rpm running.

Fig 2 is sutiable for low-load operation. Its late valve opening leads to a partial vacuum in the combustion chamber. In addition to the low valve lift, the intake air stream is greatly speeded up, generating turbulence thus improve air and fuel mixture. This benefits fuel economy and emission.

Fig 3 is suitable for a wide range of part-load operation. Depending on the requirement of power, the amount of air can be controlled by the early closing of intake valves. This eliminates the need of throttle butterfly (like BMW Valvetronic) and reduce pumping loss by up to 10%.

Fig 4 is designed for enhanced low-rpm acceleration. While it enables more intake air volume compare with Fig 2 & 3, its early valve closure ensures no air flow back into the intake manifolds near the end of the intake stroke. (Remark: the combination of fast cam timing and low rpm operation could lead to backflow, that's why Multiair needs to close the valves earlier. Other engines do not have this issue because they either use variable cam phasing or compromised cam timing)

Fig 5 is so-called "Multilift" mode and designed for very low rpm operation. It combines the strategy of Fig 2 & 3 and their benefits - regulated consumption and improved quality of air-fuel mixture.

Combining these modes, FIAT claims Multiair improves maximum power by 10%, low-rpm torque by 15% and fuel economy by 10%. Moreover, cold-start emission of HC/CO and NOx are reduced by 40% and 60% respectively due to its ability of exhaust gas recirculation. This technology is also compatitble with diesel engines, which means substantial cost reduction.

However, I can see a few weaknesses of Multiair: Firstly, at the moment it is compatible with SOHC engines only because of the bulky mechanism. This mean while it enables variable timing and lift for intake valves, it offers neither for exhaust valves. The addition of variable exhaust cam phasing may require a complex cam-in-cam mechanism like that used by Dodge Viper 8.4. Secondly, the SOHC design and the complicated electrohydraulic mechanism could generate extra friction, thus it is not suitable to high-revving high performance engines, which is a common problem shared with BMW Valvetronic. It is more suitable to mass production engines and low-revving turbocharged engines. Lastly, the electrohydraulic mechanism might complicate servicing and raise reliability issues.

Advantage Very flexible - ability to vary valve lift, early intake opening / closing and even multiple openings during an intake stroke, hence enhanced output, reduced consumption and emission. Disadvantage Mechanism quite bulky such that difficult to apply to DOHC engines; Not perfectly compatible with variable cam phasing. Who use it ? FIAT group 1.4 FIRE engine, 900cc SGE twin-cylinder engine...

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Variable Valve Timing (VVT) Part 1

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